1) Attach the system disks on the MB controller and buy a 2-port PCI-e x1 SATA card. Issue: Does anyone know the throughput of the ICH10R with Solaris ZFS in RAID5 mode?

2) Buy a 4-ports PCI-e x1 SATA card. Because I am not sure the ICH10R in AHCI mode is very fast, I am also thinking of buying a supported card with decent speed. Issue: I can't find a PCI-e x1 card that is supported and has 4 ports. This comprehensive list http://blog.zorinaq.com/?e=10 doesn't seem to have any that are compatible with my settings.

You don't necessarily need to get an add-in PCIe x1 controller as your ICH10R supports port multipliers and thus isn't limited to just 1 device per sata channel. Buy something like an Addonics AD5SAPM 5x1 Port Multiplier which sits in a empty PCI bay and will let up to five devices share a single SATA channel. Performance of these devices will be capped at single channel SATA2 max speed (300MB/sec) but that's still better than what you'd get in aggregate with any PCIe x1 device (250MB/sec).

Be aware that the Caviar 2TB Caviar Green disks have 4K sectors, but lie and report 512byte sectors to the OS. At pool creation time make sure to specify ashift=12 so that ZFS can align everything to 4K boundaries otherwise your performance will be terrible.